First post here lol. There were other shapeshifters before this pack so that would mean the first pack ran into at least one vamp. So maybe one of them got bitten and that fact was passed down along with all the other stories.

And don't forget- when Billy and Old Quil recount the legends of the spirit warriors, that pack fought two vampires and lost many of their protectors in the process. It's possible that they knew about vampire venom back then.

Well, you have to remember that we're finding out everything from Bella's POV. So, if there was no reason for her to know before a certain point then we wouldn't know. It wouldn't be a very interesting book if you found out everything at the very beginning.